1.Ross "cheated" on Rachel during their ~break~ because he was taught a different definition of the term by Carol.

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Perhaps the most enduring debate in Friends history was whether or not Ross cheated on Rachel while they "were on a break." Rachel maintained that Ross was unfaithful while he disagreed. But this theory suggests that Ross really did believe he was innocent because he was taught a different definition of the term "break" by his ex-wife Carol.

The theory is based on the fact that in one episode shortly after his divorce, Ross reveals that he lost his virginity to, and went on to marry, Carol. In a later episode, however, we learn that he also slept with a cleaning lady in college.

NBC

NBC

It's therefore suggested that Ross and Carol had a "break" some time during university, which permitted them to each hook up with other people to see what life was like without one another. Ultimately deciding that they were meant for each other, they got back together and tied the knot. Ross then assumed that the same rule applied to his "break" with Rachel.

It's well known that Ross' son Ben made regular appearances on the show until season eight when he seemingly dropped off the face of the earth, and didn't even meet his half-sister Emma. However, this theory suggests that Ben disappeared because Ross lost custody of him thanks to his irrational behaviour and questionable parenting skills.

Over the course of a few seasons Ross asked a self-defence instructor to help him scare women he was planning to attack, tried to kiss his first-cousin, dated his students, was signed off work due to uncontrollable rage and objected to employing a male nanny.

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This theory suggests that Carol decided to cut Ross out of Ben's life as a result of the erratic behaviour.

3.But he also may have just abandoned Ben to focus on his new family with Rachel.

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NBC

This theory posits that Ross didn't lose custody of Ben, but rather abandoned him in favour of his new family with Rachel. It's founded on the conversation that Rachel has with Janice in the hospital shortly after giving birth. Janice warns her that eventually Ross will leave her and Emma for his "real family" and won't be around for them. The theory suggests that in actuality, Rachel and Emma were the "real family" Ross chooses over Ben, wanting to devote his time and love to the family he's created with a woman he's loved since high school.

4.Richard knew that Chandler was planning to propose to Monica, and that's why he rushed to profess his love and commitment to her.

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One of the plot twists in Monica and Chandler's engagement episode was the reappearance of her ex-boyfriend Richard. They famously split up after establishing that Monica wanted marriage and children and Richard did not. However, right before Chandler proposes, Richard does a U-Turn and tells Monica he's never got over her and wants to commit. However, this fan theory suggests that Richard knew Chandler was going to propose and, scared of losing Monica forever, decided to do it first.

The theory is based on the fact that Chandler checks his jacket pocket to ensure the ring box is still there moments before Richard enters the same restaurant.

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When Richard arrives, he hugs Chandler and feels the box. Knowing that Chandler won't pop the question with him right there, he accepts a table next to them to buy some time, and once dinner is over, rushes to get to Monica and professes his love and commitment to her.

5.Monica had the best sex of her life with Chandler because she literally taught him how to navigate her body.

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Throughout the show, Chandler is portrayed as pretty damn useless with women — especially in comparison with lothario Joey. Despite this, when he and Monica first begin dating, she says he's the "best sex [she's] ever had." Now, if he truly were that phenomenal in bed, surely he'd have had a lot more success with women over the years?

Well, this theory explains the plot hole by referring back to the episode where Monica teaches Chandler the seven female erogenous zones, and the order in which to stimulate them.

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He likely committed this information to memory and used it on Monica when they finally hooked up. And, since the order of stimulation was specific to Monica's likes, of course it was perfect. And of course he was the best lover she'd ever had.

6.Ross and Rachel were cursed for seven years in the pilot episode, and that's why their relationship was tumultuous.

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This theory suggests that Ross and Rachel's tumultuous relationship was down to a curse triggered in the pilot episode. In the scene where Rachel meets the Friends for the first time in the coffee house, Ross' umbrella accidentally opens and the theory posits that this led them to having seven years of bad luck. The curse lifted just in time for Rachel to tell Ross she's pregnant, with the initial umbrella incident happening on 22 September 1994, and her pregnancy announcement on 27 September 2001.

7.The group always got the same table in Central Perk because Gunther secretly reserved it for Rachel.

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For many years viewers were confused as to how the Friends always managed to secure the same Central Perk couch, until someone spotted that a "Reserved" sign had been there all along. However, this didn't account for the fact that the Friends often showed up spontaneously and so hadn't always booked the spot in advance. However, this theory suggests that the sign was actually there because of Gunther's infatuation with Rachel. Since the coffee house was located in a busy area, Gunther simply left the sign on the table so that she would always get a seat and he'd be able to see her often.

8.Erica knew she was having twins but was worried Monica and Chandler would back out of the adoption if she told them.

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One of the biggest surprises in the final episode of Friends was Erica giving birth to not one but two babies. However, it seems unfathomable that Erica didn't know she was expecting twins, or that the adoption agency would've made such a huge oversight. This theory, however, suggests that Erica did know, but after making it through the rollercoaster of emotions involved in selecting Chandler and Monica as adoptive parents, and getting to know their personalities, she decided to keep the information to herself in case they pulled out. After all, Chandler did seriously freak out when he found out that there were two babies.

9.Monica, Phoebe and Rachel having very different routes into motherhood was deliberate.

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By the end of the show, all three of the female Friends had had children in one way or another. Phoebe acted as a surrogate to triplets for her brother and his wife. Rachel decided to become a single mother after a one night stand with Ross. And Monica adopted twins after being told she was biologically unable to have children. While the storylines were all dramatic, there's a theory that the writers made each experience different from the last in order to show the innumerable routes into motherhood, and how equally valid they all are.

We all know that Phoebe was kooky and ditzy. However, this theory suggests that she was actually a bona fide genius with almost zero ambition, playing one off the other for her own amusement.

The theory is mainly based on the scene in which Phoebe almost convinces Ross that the theory of evolution is flawed.

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He leaves the conversation confused and she laughs: "That was fun. Who's hungry?" However, there are other scenes in which Phoebe appears a lot smarter than she's given credit for. Remember how she referred to herself as the puppet master of the group after Rachel gave birth? Or how she tricked Chandler into admitting he was seeing Monica? Or how she attempted to get Ross and Rachel back together by setting them up on bad dates? Not to mention the fact she's able to speak fluent French despite never going to high school? Maybe she was actually incredibly smart all along.

11.Chandler deliberately tricked Monica into the quiz so that he and Joey could win her apartment.

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The trivia quiz which results in Monica and Rachel switching apartments with Chandler and Joey is one of the most entertaining episodes of the whole series. However, this theory suggests that the whole thing may have been masterminded by Chandler, who cheated in the first round knowing that he'd be able to convince Monica to bet her apartment later.

It posits that during the scene in which Joey and Chandler guess what's in Rachel's shopping bag, he steals a glance while walking over to tell Ross to stop his game show host impression.

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Seeing what's in the bag, he's then able to "guess" one of the items correctly, and genuinely suggest another using logic. Knowing how competitive Monica is, it's possible he then decided to up the stakes knowing that she'd be convinced to bet the apartment.

After all, it's him who suggests this gamble in the lightning round of the game.